Actually it would be possible to make FR-2 double sided and in the past some cheap consumer products were made of single or double sided FR-2 because the material is really cheap.

But its max temperature of 70°C is today not usable for everything concerning microelectronics. If exceeded, the copperlayer starts to lift up from the base material and cracks easily. Your PCB really should be the last thing to die (exept you intentionally use it as a fuse) if your chips are getting hot 😉
Same for vibrations and mechanical stress.

Also its surface leakage and insulation resistance is much worse than FR-4 so it would not be possible to build a working A20-LIME2 board with FR-2.

Humm? I’ve seen some old dual layer PCBs made of FR2. Though FR2 these days only being used for cheapest, lowest grade devices, which are usually ok with just 1 layer + few jumpers.

From what I know, FR2 haves some technoloigical issues when it comes to making vias. I guess there’re some issues with plating vias on such a crappy material – probably copper just does not lays properly and it does not gets reliable via connection on its own. Boards I’ve seen were using actual pieces of wire to do all layer interconnections and lacked plating in their vias.

The material FR-2 is not ductile therefore it can crack easily especially on the metalization of via under mechanical stress (or copper tracks delamination).
In addition dunno if the material could sustain the laminate process for multilayer PCB (Heat !!).